For example, Japanese Patent Application Laid-Open Publication No. 2011-130155 (Patent Document 1) discloses the process contents at the occurrence of flooding in a chassis-type switching hub in which the LAG (Link Aggregation Group) across line cards is set. Concretely, at the occurrence of flooding, a line card on an ingress side adds a distribution ID to a frame and transmits it to each line card on an egress side. Each line card on an egress side retains a table representing a correspondence relation between a distribution ID and a port ID (including line card ID), and when the line card ID corresponding to the distribution ID added to the received frame indicates the line card itself, the line card transmits a frame from a port corresponding to the port ID.